The invention relates generally to the field of railroad ballast tampers or track tamping machines and hydraulic systems for producing vibratory motion and more specifically to hydraulic vibrator systems for operating track tamping apparatus.
Prior art track tamping machines of the type to which the present invention relates are exemplified by the machine illustrated in U.S. Pat. No. 3,135,223 to Plasser et al. In this system a pair of pincer-like tamper arms are pivotally mounted on a specially designed railroad car having suitable hydraulic systems to enable the tamper arms to be driven sharply into the ballast on either side of the end of a railroad tie. The upper ends of the tamper arms are coupled by a yoke to an eccentric shaft. Rotation of the eccentric shaft imparts vibratory motion to the lower ends of the tamper arms thus assisting in consolidating the ballast. The ends of the tamper arms extend downwardly into the ballast below the tie and are gradually squeezed together by hydraulic means during the compacting operation. Hydraulic systems for controlling the distance between the two tamper arms are shown in U.S. Pat. Nos. 3,211,064, 3,146,727, 3,372,651, 3,357,366, 3,608,498, 2,872,878, and 3,669,025, all to Plasser et al and 2,791,971 to Schellmann.
As discussed in U.S. Pat. No. 3,135,223, track tamping is normally done in conjunction with a leveling operation. A rail which is found to be too low is jacked up by hydraulic means carried on the railroad car tamper unit while the ballast is compacted to raise the associated railroad tie ends which support the section of the rail. In all of the above systems, the vibration of the tamper is induced by the eccentric mounting rather than by hydraulic piston apparatus. The disadvantages which attend the use of an eccentric vibratory mechanism include the cost of replacement and maintenance of the eccentric mechanism and the overall complexity of the unit. In addition, the eccentric shaft requires a fly wheel which cannot be started and stopped between tamping operations on adjacent ties. Thus even while lifting the tamping arms to move them to the next tamping station, the vibratory motion continues, resulting in unnecessary wear, power consumption and noise pollution.
Hydraulic piston apparatus has been considered before in connection with providing vibratory motion for the tamper arms. In U.S. Pat. No. 2,973,719 to Plasser et al a double-acting hydraulic piston, gated by a hydraulic rotary distributor valve, has a rack which meshes with a pinion on a shaft to oscillate a pair of displaced jaws on the end of the shaft. The rotary valve causes six reciprocations per revolution by using a complicated double manifold arrangement and chordal throughports in the spool within the rotary valve. U.S. Pat. No. 3,735,708 to Plasser et al illustrates the use of a hydraulic piston motor which an automatic flip-flop valve to vibrate the tamping tool. The background of the U.S. Pat. No. 3,735,708 indicates the general disadvantages of using separate coaxially arranged hydraulic cylinders to vibrate tamping tools.
U.S. Pat. No. 2,022,738 to Krute illustrates a highly complicated hydraulic rotary control valve for operating a pump. The rotary valve gates hydraulic fluid to and from duplex double-acting hydraulic pistons to provide uniform output flow velocity from the pump.